Auxiliary heat treatment for aluminium-lithium alloys

ABSTRACT

Artificially aged aluminum-lithium alloys are given an auxiliary heat treatment at or after completion of ageing to improve short-transverse properties, particularly fracture toughness. The auxiliary heat treatment comprises heating the material steadily to a reversion temperature above the ageing temperature by at least 20° C. but not higher than 250° C., retaining the material briefly at temperature then cooling to room temperature. Typically the treatment involves heating to a reversion temperature in the range 190°-230° C. with a hold at this temperature of around 5 minutes. Boosted properties decay with extended exposure to temperatures of 60° C. and above but may be restored by reimposition of the auxiliary heat treatment.

This invention relates to a particular form of heat treatment foraluminium-lithium alloys, that is those alloys based on aluminium whichinclude lithium as a deliberate alloying addition rather than a traceimpurity. Practical aluminium-lithium alloys include strengtheningingredients additional to the lithium such as copper, magnesium or zinc.The heat treatment is intended for use on such alloys in certain productforms and/or tempers to improve fracture toughness or ductilityparticularly in the short transverse direction. The term "shorttransverse direction" is a term of art applied in respect of plate orsheet material to specify the axis of cross-section through thethickness of the material and used also in respect of other productforms such as extrusions and forgings to identify a cross-grainorientation.

BACKGROUND OF THE INVENTION

Aluminium-lithium alloys based on the aluminium-lithium-copper andaluminium-lithium-copper-magnesium systems have been developed to thestage where they are currently being considered for large-scalecommercial use on the next generations of civil and military aircraft.The attractiveness of such alloys as replacements for established nonlithium-containing aluminium alloy lies in their reduced density andincreased stiffness but widespread application of these materials inaerospace structures will be dependent upon attainment of a satisfactorycombination of many properties. The aluminium-lithium-copper-magnesiumalloy registered internationally under the designation 8090 providesreduced density and increased stiffness in combination with strength,fracture toughness, corrosion resistance, fatigue resistance and ease ofproduction at a level far in advance of the first aluminium-lithiumalloys. Nevertheless there remains a perceived problem with regard tocurrent aluminium-lithium alloys in regard to low fracture toughness inthe short transverse direction. It might be that a low fracturetoughness in the short transverse direction. It might be that a lowfracture toughness in this axis presents no real barrier to use of thealloys in normal applications because the materials will not besubjected to usage which presents a stress on the axis but it remainssomething of a barrier to confidence in the new materials and mightconceivably affect service life in some situations. The 8090 alloy forexample, when aged to yield a tensile strength of 500 MPa or more whichis typical of the modern high strength aerospace 7000 series alloys inthe T76 condition, can exhibit low levels of fracture toughness in theshort transverse direction typically 11 or 12 MPa (m)^(1/2) as against18 to 20 MP (m)^(1/2) for the 7000 material whilst fracture toughness inother orientations of the 8090 alloy is more than acceptable.

The problem or perceived problem is not new-found and various tentativeexplanations have been advanced previously in the prior art. It is knownthat fracture in the short transverse plane (whether crack growth occursin the longitudinal direction or the transverse direction orthagonal tothe applied stress) occurs along grain boundaries and is brittle innature showing little evidence of local ductility in those materialsexhibiting low short transverse fracture toughness. The tentativeexplanations already made in the open literature embrace the followingpossibilities: localisation of the plastic strain at grain boundaries;grain boundary embrittlement by traces of hydrogen or low melting pointmetallic elements such as sodium, potassium or calcium; and theformation of large phases at the grain boundaries containing lithium,copper and possibly magnesium. This invention provides a convenientsolution to the problem and studies made in relation to the inventionindicate that these previously proposed explanations do not go the rootof the matter though some of them relate to phenomena which will makesome degree of contribution to the problem in certain circumstances.

Those present day aluminium-lithium alloys which are produced by theingot metallurgy route rather than rapid solidification routes, aresubject to the normal processing steps used and well established in theart for other species of precipitation hardening aluminium alloys,namely: casting to ingot; homogenisation heat treatment; forming tosemi-finished product or product; solution heat treatment; quenching andartificial ageing at elevated temperature. In somealloys/tempers/products there is a cold working stage prior to theartificial ageing to secure an enhanced ageing response. The aim of theageing treatment is to promote accelerated decomposition of thepre-existing supersaturated solid solution yielding the requiredstrengthening precipitates.

Various artificial ageing treatments are known in the art in regard toaluminium-lithium alloys. Choice of ageing time and temperature permitsageing to peak strength, underageing, or overageing as required. Duplexageing treatments are known, these being treatments in which thematerial is held at first one temperature (for the first stage oftreatment) then held for a second period at a different temperature. Asfar as is known, those ageing treatments currently adopted foraluminium-lithium alloys aim to maintain the material in thermalequilibrium during each ageing period to promote a uniform precipitationof the strengthening phase or phases. We have found that the shorttransverse fracture toughness and ductility of aluminium-lithium alloysof the alunimium-lithium-copper-magnesium system can be significantlyimproved by imposition of an auxiliary heat treatment after ageing andour investigations of the phenomenon suggest that the auxiliary heattreatment will be effective also for other species of aluminium-lithiumalloys such as alloys which contain copper but not magnesium and thosealloys which contain zinc with or without copper and/or magnesium.Whilst the treatment might be expected to be of benefit to some degreein all alloy tempers it provides particularly a significant improvementin those product forms and tempers in which in the absence of suchtreatment the fracture mode would be a brittle intergrannular fracture.

Previously we had investigated the effects of a secondary ageingtreatment upon 8090 plate material in the T8771 condition (that ismaterial aged for 32 hours at 170° C.) and based on secondary ageingtimes of 1 hour or more at temperatures of 170° C. to 230° C. it wasconcluded that some slight improvement in the short transverse fracturetoughness of the material could be obtained by duplex ageing. A briefmention of this conclusion is given in a paper by C. J. Peel and D. S.McDarmaid given at pages 18 to 22 in the May. 1989 issue of Aerospace,which is the journal of the Royal Aeronautical Society. The best resultthat had been obtained by such a secondary ageing temperature was animprovement in short transverse fracture toughness as reflected by crackpropagation in the longitudinal direction (hereinafter termed S-Lfracture toughness) from approximately 20.5 MPa(m)^(1/2) to 26MPa(m)^(1/2) following ageing for 1 hour at 210° C. The practice used inthis method is typical of that used in ageing practice in that thematerial was heated and cooled slowly to achieve thermal uniformity andheld at temperature for an appreciable time in the expectation ofsecuring an ageing response.

In contradistinction to our earlier result and expectation it has nowbeen discovered that a more pronounced benefit in terms of improvementto short transverse properties can be achieved by use of a new heattreatment which is not intended to promote an ageing response and whichis different in nature to those known in the art for the purposes ofartificial ageing.

BRIEF DESCRIPTION OF THE DRAWINGS

The claimed invention is described below by way of example withreference to the drawings of which:

FIG. 1 is a graph showing a plot of SL fracture toughness againstauxiliary heat treatment times and temperatures;

FIGS. 2 and 3 are histograms illustrating the influence of heating andcooling rates; and

FIGS. 4 and 5 are histograms illustrating the benefit secured by meansof the auxiliary heat treatment on materials pre-aged to varyingstandards.

DESCRIPTION OF THE INVENTION

The invention claimed herein is an auxiliary heat treatment foraluminium-lithium alloy material applied at or subsequent to completionof ageing which comprises heating the material to increase itstemperature steadily beyond the maximum temperature attained in itsageing, hereinafter designated "t₁ ", so that the temperature exhibitedin its colder parts attains a level hereinafter termed the "reversiontemperature" and designated "t₂ ", wherein the reversion temperaturedoes not exceed 250° C. but exceeds by at least 20° C. the maximumageing temperature, retaining the material briefly at temperature butfor no more than 30 minutes to achieve thermal equilibration in thematerial, and immediately thereafter cooling the material towards roomtemperature.

The benefits of the auxiliary heat treatment are achieved throughchanges in temperature rather than holding at temperature in the mannerof an isothermal treatment and the term "steadily" as applied to theincrease in temperature achieved in the heating stage implies that thereare no deliberate holds etc in raising the temperature from t₁ to t₂. Itcould be most convenient in foundry practice to apply the auxiliary heattreatment at the end of isothermal ageing without an intervening coolingto room temperature. The heating from t₁ to t₂ is intended to beachieved as expiditiously as possible having regard to the thermalcharacteristics of the plant employed for the heat treatment and thelength of any equilibration hold at t₂ will depend of course on the massand thickness of the material and the temperature gradients imposedduring heating.

It is preferred that the material is quenched or otherwise rapidlycooled from t₂ to room temperature or therabouts. It is preferred alsothat the material is heated rapidly at least in the band between t₁ andt₂. Good results have been obtained with fast heating without fastcooling and vice versa but the best results have been obtained with fastheating followed by fast cooling. There need be no significant (if any)dwell, at the reversion temperature t₂ for the method is not intended toact in the manner of an isothermal ageing process. The best results todate have been obtained with no more than a nominal 5 minutes hold at t₂for small test piece specimens.

The preferred range for reversion temperature t₂ is 200°-230° C. alwayssubject to the proviso that t₂ exceeds to t₁ by at least 20° C.

The precise nature of the phenomenon involved in this auxiliary heattreatment is not known with certainty at present but it is believed thatheating the material above its ageing temperature in a steadynon-isothermal manner disturbs the equilibrium established within thematerial by the prior ageing causing a redistribution of grain boundarysolute elements. A new equilibrium with increased grain boundaryprecipitation might be expected to occur if the material were to be heldat the reversion temperature for an appreciable time and this conditionwould be no better than the original aged condition. Cooling thematerial prior to attainment of a new equilibrium is believed to fix thematerial in a metastable condition which exhibits the improvedproperties we have observed.

Some degree of degredation towards the original pre-treated conditionhas been found in materials exposed continuously to temperatures of 60°C. and above. It is predicted by extrapolation from measured values thatit would take 20 years of continuous exposure at 30° C. to regress tothe original condition. Re-application of the auxiliary heat treatmenthas been found to restore the degraded material to its previouscondition. It is anticipated that an application of a similar shortauxiliary heat treatment would be effective in restoring properties ofmaterial degraded by extended natural ageing or by elevated temperatureprocessing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The material used in the examples of the invention described here is8090 alloy. The compositional limits for this alloy (by weight) are asfollows:- lithium 2.2 to 2.7%; copper 1.0 to 1.6%; magnesium 0.6 to1.3%; zirconium 0.04 to 0.16%; impurities iron 0,30% maximum zinc 0.25%maximum others (chromium, silicon, manganese and titanium) 0.10 maximumeach; balance aluminium.

EXAMPLE 1

The material used for this example was 8090 plate of 2 inch thicknesssupplied in the T8771 condition. Material in this condition has beenprocessed as follows: solution treatment temperature 545° C.; quenched;stretched 7%; and aged for 32 hours at 170° C. From this plate varioustest pieces were machined suitable for measurement of fracture toughnessand tensile properties in the short transverse orientation. The fracturetoughness test pieces were of double cantilever beam form and such as togive a stressing orientation on the short transverse axis and crackgrowth on the longitudinal axis. The value of fracture toughnessobtained from these test pieces is termed herein "SL fracturetoughness". It is designated K_(Q) SL in accordance with normalmetallurgical practice to indicate that the test methodology accordswith the established rules but the crack propagation does notnecessarily proceed in a manner as required for a definative value.

Some specimens of the test material were evaluated in the as-suppliedcondition whilst other specimens were subjected to an auxiliary heattreatment prior to testing. All tests were performed at room temperatureexcept as otherwise stated. The auxiliary heat treatment was applied byimmersion of the specimens from room temperature in a salt bathpre-heated to the required reversion temperature t₂. The specimens wereheld in the salt bath (within a furnace) until they attained therequired reversion temperature as indicated by a flattening of theoutput of a thermocouple attached to a dummy specimen in the salt bath,held for a further five minutes in the bath at temperature, thenwithdrawn from the bath and quenched in cold water. Obviously theheating and cooling rates in this regime vary considerably in anon-linear manner. The overall average heating rate and cooling rate areestimated at 40° C./minute and 350° C./minute respectively. Heating andcooling in this manner are hereafter termed respectively rapid heatingand rapid cooling for the purposes of comparison. The table belowdocuments the properties of the starting material and material which hasbeen auxiliary heat treated to the above methodology at variousreversion temperatures.

    __________________________________________________________________________                0.2%                                                                     K.sub.Q (SL)                                                                       Proof                                                                              Tensile                                                                            %                                                              MPa  Stress                                                                             Strength                                                                           elongation                                                                           Reduction                                               (m).sup.1/2                                                                        MPa  MPa  to break                                                                             in area                                                                             HV.sub.10                                  __________________________________________________________________________    T8771  11.6 372  476  1.9    4.3   156                                        (control)                                                                     t.sub.2 = 190° C.                                                             18.2 356  470  3.7    8.6   151                                        t.sub.2 = 200° C.                                                             22.5 348  457  3.4    6.5   148                                        t.sub.2 = 210° C.                                                             26.0 340  447  3.7    6.25  142                                        t.sub.2 = 220° C.                                                             29.0 333  439  6.0    8.75  139                                        __________________________________________________________________________

It will be seen that the auxiliary heat treatment is extremely effectivein increasing the SL fracture toughness and ductility in the shorttransverse orientation. Some loss of short transverse strength isinvolved. The relative value of improvement and penalty might vary withthe application for which the material is intended but it is likely thatthe K_(Q) SL value can be increased to the 18-20 MPa(m)^(1/2) value ofthe 7000 series materials without incurring a limiting loss of strength.

The results of the auxiliary heat treatments documented above and thoseof other heat treatments at periods yielding isothermal ageingconditions are depicted graphically in FIG. 1. All materials documentedin this graph were treated in the same rapid heat/rapid cool regime butwith different treatment temperatures and times at temperatures. It willbe seen that there is a pronounced peak in the curve of fracturetoughness against time at temperatures which occurs in the 5 to 10minute band and that the benefits are far below the optimum with time attemperature of one hour or more. A treatment at times typical ofisothermal ageing practice impaires properties rather than improves them(insofar as reflected in K_(Q)).

Further specimens of T8771 material have been subjected to a slightlymodified form of auxiliary heat treatment which is the same as thatreported above save that the treatment involved slow heating in thefurnace in air and slow cooling out of the furnace in air. The estimatedaverage rates in these slow heating and slow cooling regimes are 4°C./minute and 400° C./hour. Some specimens were subjected to rapidheating and slow cooling and other the contrary. Results for t₂ =210° C.and t₂ =200° C. are documented in FIGS. 2 and 3 respectively. It appearsthat rapid cooling is more beneficial than rapid heating and that thebest results are obtained with rapid heating followed by rapid cooling.Useful improvements are still secured through the slow heating-slowcooling auxiliary heat treatment at the rates documented althoughwhether this improvement would be sustained with prolonged heating andcooling in the manner of isothermal ageing is uncertain.

EXAMPLE 2

For this example unaged 8090 1 inch plate in the T351 condition wasused. This material is solution treated at 535° C., quenched stretched21/2% but not aged. Using this as the starting point the material wasaged at various temperatures from 150° C. to 190° C. and for varioustimes from 4 hours to 96 hours. The artificially aged material wassubjected to auxiliary heat treatments comprising various reversiontemperatures and times at temperature. The results are documented inFIGS. 4 and 5. It will be seen that in all cases the auxiliary heattreatment secures very significant improvement in the SL fracturetoughness.

We claim:
 1. An auxiliary heat treatment for aluminium-lithium alloymaterial applied at or subsequent to completion of ageing whichcomprises heating the material to increase its temperature steadilybeyond the maximum temperature attained in its ageing so that thetemperature exhibited in its colder parts attains a level termed thereversion temperature wherein the reversion temperature does not exceed250° C. but exceeds by at least 20° C. the maximum ageing temperature,retaining the material at the reversion temperature for no more than 30minutes to achieve thermal equilibration in the material, andimmediately thereafter cooling the material towards room temperature. 2.Auxiliary heat treatment as claimed in claim 1 in which the material isquench cooled to room temperature or thereabouts.
 3. An auxiliary heattreatment as claimed in claim 1 in which the material is rapidly heatedto the reversion temperature from at least the maximum ageingtemperature.
 4. An auxiliary heat treatment as claimed in claim 1 inwhich the material is held at the reversion temperature between the heatup and the cooling down for a period of five to twenty minutes.
 5. Anauxiliary heat treatment as claimed in claim 1 in which the reversiontemperature is within the range 190° to 230° C.
 6. A material or productthereof comprising an alloy of the aluminium-lithium-copper-magnesiumsystem having improved fracture toughness in short transverse directionup to 34 mPa(m)^(1/2) having been artificially aged and then subjectedto an auxiliary heat treatment as claimed in claim
 1. 7. A material orproduct thereof as claimed in claim 6 comprising an alloy with acomposition within that specified for the registered alloy 8090 and inwhich the auxiliary heat treatment comprises a rapid heat up to areversion temperature in the range 190° to 230° C., a hold at reversiontemperature for about 5 minutes, and a rapid cool down to roomtemperature or thereabouts.